Slider contact



Nov. 21, 1961 F. P. vAcHA SLIDER CONTACT Filed July 24, 1958 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\nmmmmmunlllllll INVENTOR. FRED P, VACHA BY KENWAV4 #www4 WITrER g HILDRETH ATTORNEYS between two turns, and so on.

United States arent Ofice 3,010,090 SLIDER CONTACT Fred P. Vacha, Babson Park, Mass., assigner, by mesne assignments, to The Gamewell Company, Newton Upper Falls, Mass., a corporation of Delaware Filed July 24, 1958, Ser. No. 750,783 3 Claims. (Cl. SSS-118) The present invention relates generally to wire-wound impedance devices such as potentiometers rheostats and the like. More particularly, it relates to an improved slider contact especially adapted for yhigh lineal contact speed over the winding, and to a method for making the slider contact.

A slider contact ordinarily comprises an element fastened to an arm which rotates about an axis fixed in relation to the variable impedance winding. This element includes a contact portion that bears upon the winding and a spring portion that provides resilient bearing pressure of the contact portion upon the winding.

At low lineal contact speeds, the performance of some yforms of prior art slider contacts is adequate for most purposes. One such form comprises a two-part slider contact assembly including a flat cantilever spring secured at one end to the rotating arm, with a short length of contact wire secured to the other end of the spring for contacting the winding. Commonly, the contact wire is soldered or welded to the flat spring. ltsy diameter is ordinarily several times the diameter of the wire on the winding, with the result that the mass of the spring and contact is concentrated at the free end of the spring in Contact with the winding and the natural frequency of vibration is low.

With this form of prior art slider contact, certain undesirable characteristics are readily noted in the presence of vibrations. Such vibrations may arise from external sources which depend on the particular location of the impedance, but in most instances .they `arise principally from the movement of the contact as it travels lfrom one wire turn on the winding to the next, first moving over the top of one turn, then dropping to a low position This occurs even when the turns are wound tightly against one another, and the vibrations resulting from this movement are ordinarily )much greater than those caused by outside sources. The vibrations include not only the primary vibration of the contact wire itself, but also secondary vibrations occurring intermediate the ends of the spring, for example, midway between the ends. If large enough, the vibrations cause electrical noise which results from variations in contact resistance and interruptions in the electrical connection at the slider contact. To reduce the noise, the slider contact speed must be kept substantially below that which induces the natural or resonant frequency.

Secondly, the soldering of a contact elements is critical, in that too much solder metal further increases the mass at the end of the contact, aggravating the condition discussed above, while'too little solder tends to lead to joint failure due to fatigue.

Thirdly, the application of heat to weld or solder the parts affects the temper of the spring adversely.

The foregoing problems are typical of those which arise with increasingspeed of rotation of the slider contact to levels hitherto not considered practical. It is a principal object of this invention to provide an improved form of slider contact which exhibits very low electrical noise while giving a long service life for the Contact itself and for the variable impedancein which it is assembled.

A second and related object is to eliminate or substantially reduce the secondary vibrations discussed 3,019,090 Patented Nov. 21, 1961 `type of contact spring.

It will be recognized that a round wire winding does not present a smooth surface to the slider contact, but rather a series of crests and troughs as the contact moves from the top of a wire to the space between two turns, and so on.. Even at moderate speeds this causes the contact to vibrate, and excessive vibrations may cause a series of momentary lapses of contact, which in turn manifest themselves `as objectionable electrical noise One means ordinarily used to reduce such noise is to increase the spring pressure on the winding. However, this results in increased wear on the winding and reduces its useful life. Accordingly, a further object of this invention is to. reduce electrical noise-at high speeds without utilizing contactpressures that cause excessive wear.

With the foregoing `and other objects in view, the features of this invention comprise the formation of a novel slider contact in a single piece from round spring wire stock. The wiper is so formed that only the region closely adjacent the end at kwhich it is mounted provides the necessary resilience to establish a tolerable contact pressure, while beyond this resilient area the wiper remains in a relatively more rigid round wire form. Moreover, the wiper also incorporates an advantageous distribution of mass, whereby the concentration of mass at the end of the wiper, which is lfound in prior art devices, is eliminated. The resultant increase in the natural resonant frequency of the contact permits a higher sliding contact speed without objectionable noise. The low mass of the contact end of the wiper permits it to follow more readily the crests and troughs of its path as it travels over the winding. rl`he fact that the `contact tends to maintain electrical connection with the winding at all times during its vibrating travel prevents the electrical noise resulting from intermittent loss of such connection as experienced in the prior art.

Other features of the invention reside in certain details in the form and method of construction of the contact, as will become clear from the following description of a preferred embodiment thereof, having reference to the appended drawing, in which:

FIG. l is a partial end elev-ation of a toroidal Winding having associated therewith an improvedslider contact according to this invention;

FIG. 2 is a view showing the relation of the contact to the winding, taken on line 2 2 of FIG. 1;

FICl. 3 is a plan elevation showing details of the slider contact; and

FIG. 4 is a side elevation of the slider contact.

Referring to the drawings, there is shown a portion of a circular housing 12 to support a round mandrel 14 having an insulated surface. The mandrel has a toroidal winding 16 of impedance wire wrapped about it with the turns of the winding in close proximityy to one another. The housing has an axis 18 in which is supported a shaft 20 for rotation about the axis at a desired speed which may be constant or variable. A slider arm member 22 is supported on the shaft 20 on an insulating sleeve 24.

At the end of the arm member 22 is soldered or welded the improved slider contact designated generally at 26.

etails of the construction of the contact are shown in FIGS. 3 and 4.

The slider contact 26 is fabricated of round spring wire stock in a single piece. A spring portion 28 is irst formed by flattening the wire at one end. This portion is relatively short preferably making up less than half the finished length of the contact. Beyond the spring portion, the wire in its round form is then provided with reverse bends 30 forming an intermediate portion 31 and terminating in a contact portion 32.

The contours and manner of formation of the contact portion 32 from the round wire stock have been'found to be of great importance. By means of suitable cold dies the contact portion 32 is deformed, preferably by cold Working, so as to produce a contact surface 34 of large radius, backed by a substantially dat face 36. The curvature of the surface 34 is preferably substantially circul-ar -in shape to produce a portion of a cylindrical surface with a radius of curvature substantially greater than the radius of the round spring Wire, and particularly that of the impedance Wire on the winding 155. For example, the radius of the contact curvature may be fty times that of the impedance wire. in addition, as shown in FIG. 3, the plane of .the at portion 36 is formed at an acute angle to that of the surface of the spring portion 28.

When assembled on the arm member 22 (FIG. l) by soldering the end of the spring portion 28 thereto as indicated in FIG. 2, the curved portion 34 of the slider contact bears upon the Winding 16 with the flat surface 36 substantially parallel to the tangent passing through the path of travel of the wiper contact at the point of contact. The axis of the curved surface 34 is parallel with the axis 1E. lt will be understood that in place of having -the slider contact move over the inner diameter of the Winding, it may be supported to move over the outer diameter or either side thereof by properly orienting the end of the arm 22 to which the contact is soldered.

In operation, as the speed is increased, little increase in electrical noise is observed as compared with prior art devices, because of the remoteness of the slider contact 26 from mechanical resonance. Furthermore, the mass of the contact portion 32 is low compared with that of the spring portion 28, thereby permitting it readily to follow the crests and troughs between the turns of the winding 16. The contact pressure is not excessive and does not cause undue Wear on the winding 16, since objectionable electrical noise is substantially eliminated through the favorable mass distribution. As a results, the Winding 16 does not wear at an excessive rate and the 4service life of the potentiometer is relatively long.

The foregoing one-piece contact spring wire construction provides a potentiometer which is resistant to severe shock and vibrations, due to the absence of any soldered or Welded joint and resultant low mass at the p oint of maximum vibrations. The reduction in the number of soldered or welded joints also reduces the number of potential service failures. Also, the temper of the contact portion of the Wire is not impaired by the heat required for Welding or soldering contact Wires in prior art dev1ces.

The dies that Work the surfaces 3 4 and 36 into the end 32 of the contact member impart improved properties to the Wearing surface thereof through Work hardening. The working of the curved surface 34 into the Wire has the further advantage, especially in precision potentiometers, of removing or substantially smoothing out the tool marks ordinarily made in drawing the Wire stock. The smooth curve and surface of the portion contacting the Winding combine, with the proper locationy ofthe Wiper in relation to the Winding, to reduce the rate of Wear on the latter. 4

It will be seen from the foregoing that an improved one-piece spring Wire slider contact, well adapted for high speed use and precision application, has been provided. While the invention has been described with reference t0 a specific embodiment, various designs including obvious modifications therein will occur to those skilled in this art, utilizing the teachings hereof without departing from the spirit or scope of the invention.

Having thus described the invention, l claim: y

l. In a Wire-wound variable impedance, a slider contact formed of a single piece of spring Wire, said Wire having afiat, resilient portion at one end for flexibly mounting, a contact portion at the other end and a substantially rigid intermediate portion of round cross-section, the contact portion having its principal dimension substantially at right angles to the resilient portion and substantialrly in the plane thereof, the contact portion further having a substantially cylindrical surface for contact with the impedance, the radius of curvature of said surface being greater than that of said intermediate portion.

2` In a Wire-wound variable impedance, a slider contact formed of a single piece of spring Wire, said Wire having a dat, resilient portion at one end for flexibly mounting, a contact portion at the other end and a substantially rigid intermediate portion of round cross-section, the contact portion having its principal dimension substantially at right angles to the resilient portion and substantially in the surface plane thereof, the Contact portion further having a substantially fiat side forming an angle with said plane and a substantially cylindrical side for contact with the impedance, the radius of curvature of said cylindrical side being greater than that of said inter-mediate portion,

3. A variable impedance having, in combination, a mandrel formed in a circle, a Wire Winding over the mandrel, a shaft supported on an axis through the center of said circle, an arm rotatably supported on the shaft, and and a slider Contact consisting of a single piece of spring Wire having a flat, resilient portion at one end flexibly mounted to the arm, a rigid contact portion at the other end and a substantially rigid intermediate portion of round cross-section, the contact portion having its principal dimension substantially at right angles to the resilient portion and substantially in the surface plane thereof, the contact portion further having a substantially at side forming an acute angle with said plane and a substantially cylindrical side in contact with the winding, the radius of curvature of said cylindrical side being greater than that of said intermediate portion.

References Cited inthe file of this patent UNTTED STATES PATENTS 432,894 Parrish July 22, 1890 2,534,356 Kramer Dec. 19, 1950 2,737,560 Macher Mar. 6, 1956 2,798,137 Rasmussen July 2, 1957 2,799,756 Graham July 16, 1957 

